1. Field of the Invention
The present invention relates to a thermoelectron generating source for generating thermoelectrons by an indirect heating method including that is employed to suppress a charge-up on the surface of a substrate in radiating an ion beam, and an ion beam radiating apparatus having such thermoelectron generating source by radiating an ion beam to the substrate to perform the ion implantation or other treatment.
2. Description of the Related Art
In an ion implantation apparatus, if an ion beam is applied onto a substrate of irradiated object, positive charges of ions making up the ion beam are accumulated on the surface of the substrate to produce a charge-up (electrification). If this charge-up is left away, a semiconductor device formed on the surface of the substrate breaks down dielectrically.
To prevent this dielectric breakdown, a variety of techniques for suppressing (relieving) the charge-up using electrons have been already offered. One of the techniques involves producing a plasma and employing electrons in the plasma (e.g., refer to patent document 1).
[Patent Document 1]
                JP-A-3-93141 (left lower column at page 2, right lower column at page 2, FIG. 1)        
With this technique, a small plasma source is placed near the substrate, electrons of low energy in a plasma produced by this plasma source are led together with the ion beam to the substrate to neutralize positive charges of ions with the electrons, thereby suppressing the charge-up.
However, the technique employing the plasma source has the following problems: (a) the energy distribution of electrons in the plasma led out from the plasma source has a range from about 10 eV to about 20 eV, which is still too high, and (b) a gas required for plasma production flows out of the plasma source into a beam line to aggravate the degree of vacuum.
Particularly in the problem (a), if electrons are excessively supplied to the substrate, a negative electrification is caused on the substrate by the electrons, so that an electrification voltage on the surface of the substrate is increased to a voltage (e.g., about 10 to 20V) corresponding to the energy of electron. In recent years, there is a great demand for suppressing electrification on the surface of the substrate to make the electrification voltage on the surface of the substrate lower. For example, when a semiconductor device is produced by radiating ion beam for ion implantation into a semiconductor substrate, there is a demand for suppressing the electrification voltage for ion implantation to a lower value (e.g., about ±6V or less) to prevent dielectric breakdown of the semiconductor device, because the semiconductor device is micro structured in recent years, but the conventional technique employing the plasma source can not meet this demand.
In order to solve the above-mentioned problems (a) and (b), there is a technique for suppressing the charge-up by generating thermoelectrons from a thermoelectron generating source of the indirect heating.
This technique involves heating a facial main cathode, employing a filament provided behind it, emitting a large amount of thermoelectrons from the main cathode, extracting the thermoelectrons through an extraction electrode, conducting the thermoelectrons together with an ion beam to the substrate, and neutralizing positive charges of the ion beam with the thermoelectrons to suppress the charge-up.
With this technique employing the thermoelectron generation source of the indirect heating type, the potential of the main cathode is set to be lower (e.g., in a range from 0V to −6V) than that of the substrate holder, whereby a large amount of thermoelectrons having a low energy from 0 eV to 6 eV are generated for neutralization to suppress the negative electrification voltage of the substrate to 6V or less. Hence, it is possible to suppress the charge-up of the substrate caused by ion beam radiation to be low.
However, the above thermoelectron generation source of the indirect heating type has a problem that the energy of thermoelectrons led out of the extraction electrode is very low, and a large amount of thermoelectrons are generated to stay (accumulate) near the front of the extraction electrode, so that extracting the thermoelectrons from the extraction electrode is prevented (limited) by negative space charges (negative potential) of the thermoelectrons.